Methods of Analysis of OCT in RVO

There are several ways to analyze OCT data. A brief review of relevant terms, the various methods of analysis, and the pros and cons of the methods follows.

Retinal Thickness Ð the value in microns of the distance between the OCT layer selected by the segmentation algorithm to represent the outer retinal boundary and the internal limiting membrane. All OCT devices use the internal limiting membrane as the inner retinal boundary, but they vary in their choice of an outer retinal boundary, which inßuences the retinal thicknesses obtained.

Retinal Thickening Ð the calculated value equal to the thickness minus the population mean for the variable under consideration (either CPT or CSMT). The normative value chosen should be speciÞed as these values differ according to source.13,17,61

Center Point (CP) Ð in time-domain OCT, the intersection of the six radial scans of the fast macular thickness protocol of the OCT.

Center Point Thickness (CPT) Ð in time-domain OCT, the average of the thickness values for the six radial scans at their point of intersection.

Central Subfield (CS) Ð the circular area of diameter 1 mm centered around the center point. Central Subfield Mean Thickness (CSMT) Ð the mean value of the thickness values obtained

in the central subÞeld.

Absolute Change in Thickness Ð the difference in the thickness between two measurements made at different times. For example, if measurements M1 and M2 are made at two different times, then the absolute change in thickness equals M2 − M1. The absolute change in thickness is equal to the absolute change in thickening, which is the Þrst of three methods of analyzing OCT changes listed above.

Relative Change in Thickness Ð the absolute change in thickness divided by the baseline thickness. Using the symbols introduced previously, relative thickness equals [(M2 − M1)/M1] á 100%, which is the second of three methods of analyzing OCT changes.

Relative Change in Thickening Ð the absolute change in thickness (or thickening) divided by the baseline thickening. Using the symbols introduced previously, relative change in thickening equals [(M2 − M1)/(M1 − normative mean)] á 100%, which is the third of three methods of analyzing OCT changes.

Different names for the various methods appear in the literature. For example, the relative change in thickness method is also termed the variation ratio method,107 and the relative change in retinal thickening method is also called the standardized change in macular thickness method.17

CPT and CSMT are highly correlated in diabetic macular edema (correlation coefÞcient greater than or equal to 0.98) and presumably in RVO with ME; therefore, conclusions derived from analyses based on CPT and CSMT should be equivalent.14 Because CSMT is derived from more measurements, is less dependent on centration of the OCT scan, and has a smaller coefÞcient of repeatability, CSMT is the preferred measurement.14

In analyzing data sets with small degrees of retinal thickening, analysis of the absolute change in thickness may be the preferred method because of its stability. In particular, analysis using relative change in thickening should probably be avoided because of the instability of this measure in these circumstances. On the other hand, when baseline retinal thickening is large (e.g., greater than 400 m), then analysis based on relative change in thickening is the better choice because it tends to control for the variable clinical importance of an observed absolute change in thickness depending on the baseline thickness. For example, a 150-m decrease in macular thickness is striking in a macula with a baseline thickness of 350 m and of minor clinical signiÞcance in a macula with a baseline thickness of 750 m.14